Although human society has progressed significantly over past centuries through the development and use of petroleum-derived products (e.g. fuels, plastics, solvents, etc.), their over-utilization has caused environmental issues including increasing atmospheric concentration of CO2 (a greenhouse gas), pollution from petrochemical production and use, and disposal of non-biodegradable plastic materials. More importantly, petroleum resources are finite and not renewable in nature. For these reasons it is necessary to seek alternative approaches to produce fuels and chemicals using renewable resources. Photosynthetic cyanobacteria have attracted significant attention in recent years as a ‘microbial factory’ to produce biofuels and chemicals due to their capability to utilize solar energy and CO2 as the sole energy and carbon sources, respectively.
Lipid-rich cyanobacteria and microalgae have most notably been employed to produce fuels such as biodiesel. Cyanobacteria are also natural producers of the naturally-occurring biodegradable plastic poly-β-hydroxybutyrate (PHB). Despite efforts to enhance PHB biosynthesis through both genetic engineering strategies and the optimization of culture conditions, PHB biosynthesis by cyanobacteria was a multi-stage cultivation process that involved nitrogen starvation followed by supplementation of fructose or acetate, which does not capitalize on the important photosynthetic potential of cyanobacteria. Most importantly, as neither lipids nor PHB are secreted by the cells, the required processes for their extraction are energy-intensive and remain as one of the major hurdles for commercial applications. As a result, researchers have recently focused on engineering cyanobacteria to instead produce secretable biofuels and chemicals. However, most production titers are below 200 mg/l and to our knowledge no report demonstrated the potential of employing photosynthetic microorganisms in a continuous production process.